1. Field of the Invention
The present invention relates to a method of blackening treating a surface or surfaces of a stainless steel strip, e.g. a cold-rolled strip, by depositing soot uniformly and stably on the surface in a treating furnace installed separately from and upstream of a continuous annealing furnace for continuous annealing of the strip.
2. Description of the Related Art
Generally, a cold-rolled stainless steep strip is annealed by being passed along a series continuous annealing line comprising, for example an annealing step and a pickling step, in order to eliminate processing strain or the like caused by rolling. Such annealing has been provided widely by an open-air continuous annealing furnace such as of the horizontal type (catenary type). Such continuous annealing furnace is designed to heat the stainless steel strip mainly by direct ignition burners, and therefore the stainless steel strip is heated chiefly by radiation heat. However, since the surface of the stainless steel strip product is required to have a gloss and uniform finish after annealing, the strip is cold-rolled in a cold rolling step before annealing in order to provide a substantial or advanced surface gloss. As a result of such high surface gloss of the stainless steel strip, the rat of head absorption during annealing is extremely low, and it is difficult to obtain high production efficiency of the annealing process. Accordingly, in order to raise the production efficiency of the annealing process, there have been attempted a method of preheating by combustion waste gas in forced convection or preheating of combustion air, a method of increasing radiation heat by raising the annealing furnace temperature over the heating temperature of the stainless steel strip to increase the temperature difference between the material temperature and the furnace wall temperature, a method of improving heat transfer efficiency by direct contact of a high temperature burner flame with the stainless steel strip, and combinations of such methods. It also has been proposed to raise production efficiency by extending the length of the heating run or zone of the annealing furnace.
These methods, however, involve the following problems. First of all, in the method of heating by increasing the radiation heat by raising the temperature difference between the material temperature and the furnace wall temperature, the speed of movement of the stainless steel strip to be heated continuously varies, or quantity of the heat transferred to the stainless steel strip becomes uneven due to contamination of the surface of the stainless steel strip or the like, or abnormality of the material may be caused by extremely exceeding the desired material temperature, or in a worst case the stainless steel strip may be melted down in the annealing furnace, among other troubles.
In the heating method by direct contact of burner flame with the stainless steel strip, such method is effective if the temperature of the stainless steel strip is low. However, when the temperature of the stainless steel strip exceeds a certain range, the surface of the stainless steel strip may be extremely and locally oxidized, or if the speed of movement of the stainless steel strip varies, the temperature of the stainless steel strip can be increased extremely to produce material abnormality.
In the method of extending the heating zone of the continuous annealing furnace, in a new furnace an increased equipment cost is required for the portion of extension, or in an existing furnace it takes much time and cost for modification. Further, after extension of the heating zone of the annealing furnace, the basic unit of the fuel is increased and heating efficiency is lowered.
Accordingly, as a method of enhancing the rate of heat absorption of the stainless steel strip surface without sacrificing the desired properties and quality such as gloss of the stainless steel strip surface and without causing other difficulties, a method of blackening treatment of the surface of a stainless steel cold-rolled strip with soot at the upstream side of the radiation heating zone of the annealing furnace was proposed in Japanese Unexamined Patent Publication (KOKAI) No. JP-A 1-119628 (1989). In such conventional method of blackening treatment of the stainless steel strip surface, plural soot generation burners are needed to blacken the stainless steel strip surface uniformly with soot. If the soot is deposited on the surface of stainless steel strip by burning the fuel with plural soot generation burners, the soot is not deposited uniformly. Thus, in the subsequent continuous annealing furnace the rate of heat absorption is not always raised, and fuel consumption is increased, as confirmed experimentally by the present inventor.
The reasons are as follows. Since the rate of heat absorption of soot has a specific value, if the soot is deposited over a long period of time, the rate of heat absorption is not raised above a certain value. Since the soot is generated in the following steps from the hydrocarbon gas of the fuel, soot progressed up to the oxidation step no longer contributes to the rate of heat absorption because its adhesion to the stainless steel strip is reduced extremely, and the deposited soot easily is peeled off or is vaporized to be in a non-blackening state due to convection in the forced convection preheating zone or heating zone at the upstream side of the continuous annealing furnace in which the strip is passed after the blackening treatment:
(Pyrolysis) --- (Generation of nucleus) --- (Surface growth, combining) --- (Grouping) --- (Oxidation)
Therefore, unless controlled so as not to progress up to the oxidation step by suppressing the soot generation step within the soot grouping step at the exit side of the blackening treating furnace, depending on the speed of movement of the stainless steel strip, it is impossible to deposit uniformly and stably on the surface of the stainless steel strip soot which adheres smoothly to the stainless steel strip and is not easily peeled off or vaporized and is capable of obtaining sufficient heat absorption.
The soot generation burner is supplied with oxygen, air or oxygen-enriched air containing 0.3 or less of the quantity of oxygen necessary to combust completely hydrocarbon gas of the fuel. This generates soot, but it is necessary to feed combustion air properly in order to progress while controlling the soot generation step. It is necessary to control the furnace temperature to be at a relatively low temperature. If the incomplete combustion flame of the hydrocarbon of the fuel injected from the soot generation burner toward the stainless steel strip surface burns at a low air ratio as mentioned above, since the majority of the inside of such flame is composed of incomplete combustion flame of relatively low temperature while the outside thereof is a high temperature complete combustion flame, it is necessary to lower the combustion temperature by injecting secondary air of relatively low surface temperature from a secondary air nozzle toward the stainless steel strip surface in order to lower the temperature of the outside complete combustion flame. That is, the rate of combustion reaction of the hydrocarbon gas in the fuel, or the rate of soot generation, varies with the low temperature secondary air volume injected from the secondary air nozzle, and the temperature of the furnace atmosphere changes accordingly. However, if the volume of low temperature secondary air injected from the secondary air nozzle is increased, the combustion reaction is promoted, and the flame temperature of the incomplete combustion flame goes up. As the flame temperature elevates, the temperature of the furnace atmosphere rises. Hence, the combustion reaction speed increases, soot generation decreases, and the deposit of soot becomes unsatisfactory. Thus, it is difficult to control the rate of combustion reaction of the hydrocarbon gas of fuel, that is, the rate of soot generation, only by controlling the volume of secondary air injected from the secondary air nozzle or its temperature.